1. Field of the invention
The present invention relates to an image exposure apparatus, and more particularly, to an image exposure apparatus having a semiconductor laser drive circuit which stabilizes the light output of a semiconductor laser element.
2. Description of the Prior Art
In a conventional image exposure apparatus, a photosensitive body is scanned and exposed by a laser light beam emitted from a semiconductor laser and the laser beam is modulated based on an image signal. The semiconductor laser has a characteristic that the laser beam is emitted when a driving current is applied which is over a threshold level. The intensity of the laser beam can vary significantly due to fluctuations in the threshold current level, as shown in FIG. 8. These fluctuations are caused in part by changes in the ambient temperature and occur over time, in part by inherent variations in the temperature response of each individual laser element and in part by age deterioration effects. Therefore, the intensity of the laser beam will vary over time even when being driven at a constant threshold current level. This stability in the laser light beam intensity causes blurring and a collapsed image shape. Consequently the image quality is greatly deteriorated. Therefore, the strength of the laser light beam to be output by the output control device of the image exposure apparatus must be stabilized in an environment where changes occur in the ambient temperatures of the semiconductor laser element, where individual laser elements have slightly varying response curves, and where the response curve of the laser element changes as the laser element ages.
In the prior art apparatus, the strength of laser light is stabilized in the following manner. After adding a threshold current as an offset current to a modulated signal, the drive circuit drives the semiconductor laser element by the added modulated signal. When the laser light beam is not scanning the photosensitive drum, a monitor circuit monitors the output of the semiconductor laser to provide a feedback signal to the drive circuit in order to control the offset current.
However, the semiconductor laser element has the characteristic that it is easy to destroy the semiconductor if the driving current should become much greater than the threshold current level. This overcurrent should be kept as close as possible to the threshold current level. Therefore, this prior art apparatus controls the drive current conventionally so that it provides the semiconductor laser element with a drive current which is just large enough to cause the laser element to begin emitting. Considering the difference of the semiconductor laser element and the range of environmental temperature, the drive current is increased gradually to generate a predetermined light strength in the laser device. The predetermined light strength is determined when the semiconductor laser element has just begun emitting.
Therefore, the prior art apparatus has a disadvantage that it has to cause the semiconductor laser element to emit sufficiently in advance of scanning, because it takes a long time to stabilized the strength of laser light beam under some circumstances. Moreover, the gain of the control loop cannot be excessively enlarged to ameliorate the above-mentioned problem, because the image quality is occasionally ruined. Also, if the gain of the above-mentioned control loop is enlarged, the above disadvantage is solved but the control in the image exposure area becomes unstable in some respects, such as the over contact heat characteristic. Therefore, determining the gain of the control loop was very difficult.